Environment Monitoring System for a Vehicle

ABSTRACT

The invention relates to an environment monitoring system ( 3 ) for a vehicle ( 1 ), wherein the environment monitoring system ( 3 ) comprises: at least two distance sensors ( 4 - 1, 4 - 2, 4 - 3 ) for detecting distance by measuring the propagation time of detection signals, wherein the distance sensors ( 4 - 1, 4 - 2, 4 - 3 ) are each designed as a transmitting unit and receiving unit for the detection signal and, in a direct operating mode, emit detection signals, receive reflected components of the detection signals emitted by the distance sensors; and emit active measurement signals according thereto; and a control device ( 5 ), which receives the measurement signals of the distance sensors ( 4 - 1, 4 - 2, 4 - 3 ) and determines an object distance (s) of detected objects ( 10 ). According to the invention, at least one distance sensor ( 4 - 1, 4 - 2, 4 - 3 ) additionally can be operated in an indirect operating mode in order to detect a detection signal emitted by another distance sensor ( 4 - 2, 4 - 3, 4 - 1 ) and reflected by the object ( 10 ) and in order to generate an indirect measurement signal.

The invention relates to an environment monitoring system for a vehicle and to a method for determining an object distance.

Environment monitoring systems on vehicles are used to determine objects in the vehicle environment. In the case of rear area monitoring systems, the intention is to determine, in particular, a possible collision with objects in the rear area (environment behind the vehicle) during reversing.

For this purpose, the environment monitoring systems have distance sensors. During propagation time measurements, a distance sensor emits a detection signal into the region to be monitored at an emission time. If an object is detected by the detection signal, the object reflects back said signal, with the result that the distance sensor can detect it at a receiving time. The propagation time of the detection signal can be determined as the difference between the receiving time and the emission time, with the result that the total distance which represents twice the distance between the object and the sensor can be determined using the signal speed. Such propagation time measurements are carried out, in particular, with ultrasonic sensors and radar sensors, to some extent even with light beams (laser) as the detection signals. Direction-dependent or angle-resolved detection cannot be carried out with propagation time measurements for the time being.

DE 10 2007 052 977 A1, for example, proposes triangulation in which two ultrasonic sensors are arranged in a horizontal line in the bumper region of a vehicle and each separately carry out a propagation time measurement, with the result that two items of distance information are determined and can be used to determine the distance between the sensor and the vehicle or the environment monitoring system by means of triangulation, such a distance generally being determined as the minimum distance from the vehicle. DE 10 2006 002 232 B4 also proposes such triangulation for determining the position of an object by measuring two distances from two different positions.

During such triangulation processes, a triangle can thus be determined given the known sensor distance (distance between the sensors) and the separately determined individual distances between an object and each of the two distance sensors, with the result that the distance between the object and the monitoring system is determined as the height in this triangle. DE 10 2007 042 220 A1 also proposes such triangulation using ultrasonic sensors. DE 41 37 068 A1 provides an integrated optical multiple distance sensor which proposes optical triangulation using position-sensitive diodes.

DE 195 07 957 C1 proposes triangulation using infrared LEDs, a road surface being scanned in order to detect a lane boundary. DE 102 51 357 A1 discloses a method for setting or switching off a travel direction indicator, in which lane and/or travel direction changes are determined from the environmental data determined, a distance measurement as a propagation time measurement using infrared sensors of a mono camera and the distance measurement using triangulation of a stereo camera also being described in addition to lane detection.

Such triangulation methods presuppose that each distance sensor detects the object to be determined at substantially the same location and a triangle is thus formed. However, such determination by means of triangulation may be more complex in the case of larger objects. Furthermore, one of the distance sensors may possibly also not detect a measurement signal if, for example, the object has oblique surfaces running in an unfavourable manner, since radar beams and also ultrasonic waves undergo directed reflection, with reflections on unfavourable oblique planes possibly not resulting in an echo at the distance sensor. The large number of distance sensors, usually six to eight distance sensors for a vehicle width of a commercial vehicle of 2.5 m, for example, which is generally required for conventional rear area monitoring systems is also disadvantageous.

The invention is based on the object of providing an environment monitoring system which makes it possible to reliably monitor the environment with relatively little effort.

This object is achieved by an environment monitoring system according to Claim 1. A corresponding method for determining distances of objects, in particular using such an environment monitoring system, is additionally provided. The subclaims describe preferred developments.

According to the invention, an indirect measurement is therefore carried out, in which a first distance sensor emits a detection signal and another distance sensor, without transmitting, detects the detection signal from the first distance sensor that is reflected by an object, that is to say receives an indirect echo, in a passive operating mode.

The detection signal output by the first, transmitting distance sensor thus reaches the object over a first distance, is reflected at the object and reaches the second distance sensor over a second distance. This indirect measurement therefore makes it possible, during propagation time determination, to determine total distances which are the sum of the distances between the transmitting and receiving distance sensor and the object.

Conventional direct measurements, in which a distance sensor transmits and receives, are preferably combined with the indirect measurements according to the invention, thus forming a combined operating mode. With this combination, both distance sensors therefore receive, while only one transmits. A direct measurement for determining the first distance and the indirect measurement of the combined total distance, from which the other distance can be determined using the measured first distance, are therefore carried out.

Such combined operating modes can advantageously be alternately carried out, with the result that each distance sensor alternately transmits and the others receive.

A few advantages are achieved according to the invention:

Indirect measurements can be carried out in addition to the direct measurements without relevant additional outlay on hardware; only additional software programming of the distance sensors is required to the effect that they can be operated in the passive receiving operating mode without their own transmission signal.

The indirect measurement makes it possible to carry out triangulation even if one of the two sensors possibly does not receive a direct measurement, which may exist, for example, in the case of unfavourably running reflection surfaces of the object to be detected. In such cases, the distance between both distance sensors and the object and additionally also the lateral position of the object can nevertheless be determined as a result of the combined operating mode.

The indirect measurements can be carried out, in particular, in addition to the direct measurements in order to enable mutual plausibility checking or estimation of errors. This is already helpful when using only two distance sensors. In such a case, the distance sensors may be fitted, in particular, to the lateral regions of the vehicle rear and may have relatively wide emission angles, for example more than 60 degrees, preferably almost 90 degrees, in order to largely detect the rear area in each case, with the result that the overlapping region of the emission angles is large. The emission angles are directed inwards, in particular, with the result that they detect the entire rear area to the rear as the overlapping region.

The indirect measurement should, in principle, result in the same total distance as the sum of the individual distances for two distance sensors in both measuring directions, with the result that the two mutual, indirect measurements can also be used for verification.

According to the invention, additional operating modes or detection methods may be carried out, in particular also for the case in which an object to be detected does not result in an echo in the two sensors. These additional detection methods may comprise in this case:

-   -   a radius estimation if only a direct echo from an individual         distance sensor has been received,     -   a combination of the direct echoes for conventional         triangulation,     -   in the case of only indirect measurements, the determination of         the object distance on an ellipse. The formation of such an         ellipse may, in principle, be sufficient to detect a minimum         possible distance;     -   a back-up function or additive power function in which both or         all distance sensors simultaneously transmit and receive, thus         making it possible to superimpose all emitted detection signals         in order to increase the total power. In this case, although the         distance of the object can only be estimated, a wavefront which         comes relatively close to the horizontal line of the sensor         system already results when a plurality of distance sensors are         used. The total power achieved thereby therefore makes it         possible to detect objects, which may not yet be possible by         means of individual direct measurements.

The invention therefore provides a cost-effective system which, in addition to the known direct measurements and direct triangulation methods, enables additional operating modes and detection methods which considerably improve the reliability and detection accuracy and the plausibility checking of measurement results without relevant additional outlay on hardware for this purpose.

The distance sensors can be synchronized for the indirect measurements by the common control device which is also required anyway to determine data in the conventional triangulation methods. In this case, synchronization signals may be output via a suitable bus system or via a star connection; for example, a LIN bus may be provided between the distance sensors and the common control device. The control device may output the synchronization signals as bus commands, in which case all distance sensors are addressed, for example, and the statement of the respective transmitting distance sensor is contained as a parameter. The measurement signals are accordingly output by the distance sensors to the control device via the bus. With such a design, the additional outlay on software is relatively low.

The invention is explained in more detail below using some embodiments on the basis of the accompanying drawings, in which:

FIG. 1 shows a vehicle having an environment monitoring system according to the invention in accordance with a first embodiment with two distance sensors, and a schematic illustration of the distance measurement;

FIG. 2 shows a schematic illustration of the measurement principle according to the invention and the signals;

FIG. 3 shows an alternative embodiment to FIG. 1 with three distance sensors.

A vehicle 1 may be in the form of a trailer vehicle or else an individual vehicle, for example. Fitted in or to its rear region 2 is a rear area monitoring system 3 which, in the embodiment according to FIG. 1, has two ultrasonic distance sensors 4-1, 4-2 and a control device 5 which are connected to one another via a LIN bus 6, with the result that the rear area monitoring system forms a bus system.

The two ultrasonic distance sensors 4-1 and 4-2 are arranged on the lateral outer regions of the rear region 2; according to the plan view in FIG. 1, the left-hand distance sensor 4-1 is thus arranged on the very left and the right-hand distance sensor 4-2 is arranged on the very right of the rear region 2 of the vehicle 1.

The ultrasonic distance sensors 4-1 and 4-2 have, for example in a manner known per se, a diaphragm which is used both to transmit and to receive ultrasonic waves. Alternatively, however, the ultrasonic distance sensors 4-1 and 4-2 may also each have separate transmitting and receiving devices.

FIG. 1 shows emission angle regions 8-1 and 8-2 of the distance sensors 4-1 and 4-2 which detect a rear area 7 behind the vehicle 1; these emission regions 8-1 and 8-2 may be emission cones, for example, but there is advantageously an emission characteristic substantially in a horizontal plane. In FIG. 2, the ultrasonic waves emitted by the first ultrasonic distance sensor 4-1 are denoted using 9-1 and the ultrasonic waves then reflected by an object 10 are denoted using 11-1. Accordingly, the ultrasonic waves emitted by the second ultrasonic distance sensor 4-2 into its emission region 8-2 are denoted 9-2 and the ultrasonic waves then reflected by the object 10 are denoted 11-2. The emission regions 8-1 and 8-2 are each directed rearwards and inwards, with the result that the emission regions 8-1 and 8-2 largely overlap. The detection regions of the ultrasonic distance sensors 4-1 and 4-2, within which they can receive reflected ultrasonic waves, are generally larger than their emission angle regions 8-1 and 8-2.

According to the invention, a first, direct operating mode is possible, in Which—in a manner known per se—each distance sensor 4-1 and 4-2 actively separately emits ultrasonic waves 9-1 and 9-2 and then detects its reflected ultrasonic waves. The first ultrasonic distance sensor 4-1 thus emits, in this first operating mode, ultrasonic waves 9-1 which are partially reflected by the object 10 as ultrasonic waves 11-1 and detects these reflected ultrasonic waves 11-1 after a time difference ΔT. A distance L1 between the object 10 and the first distance sensor 4-1 can then be detected according to the principle of propagation time measurement: the ultrasonic waves 9-1, 11-1 cover the distance 2×L1 at the speed of sound c, with the result that

2×L1=ΔT×c,

from which L1 can be determined. The distance sensor 4-1 passes a direct measurement signal S1 to the control device 5.

Accordingly, the second ultrasonic distance sensor 4-2 actively measures its distance L2 from the object 10 in the direct operating mode by means of a propagation time measurement and passes a direct measurement signal S2 to the control device 5.

Furthermore, the distance d between the distance sensors 4-1 and 4-2 is known, with the result that the triangle 4-1, 10, 4-2 is known completely with its sides L1, L2 and d, and an object distance s thus results as the height in this triangle, the height s being perpendicular to d. The object distance s is thus determined in the control device 5 by means of triangulation using the known triangle 4-1, 10, 4-2.

According to the invention, a second, indirect operating mode is also possible, in which the distance sensors 4-1 and 4-2 receive reflected ultrasonic waves 11-2 and 11-1 which were emitted by the respective other distance sensor 4-2 or 4-1: the first distance sensor 4-1 thus emits ultrasonic waves 9-1 and the second distance sensor 4-2, without transmitting, passively detects the ultrasonic waves 11-1 reflected by the object 10. In this second operating mode, the ultrasonic waves have therefore covered the total distance L1+L2 from the first distance sensor 4-1, via the object 10, to the second distance sensor 4-2. By virtue of the fact that the distance sensors 4-1 and 4-2 are synchronized, the time difference between the sending time in the first distance sensor 4-1 and the receiving time in the second distance sensor 4-2 can be determined as the propagation time and the total distance L1+L2 can accordingly be calculated using the speed of sound c.

Furthermore, conversely, the second distance sensor 4-2 may also actively emit ultrasonic waves 9-2 and the first distance sensor 4-1, without transmitting, can accordingly passively detect the second ultrasonic waves 11-2 reflected by the object 10, with the result that the same total distance L2+L1 can be determined by means of the propagation time measurement.

The direct and indirect operating modes are advantageously combined to the effect that one distance sensor, for example 4-1, transmits and itself firstly receives in the direct operating mode and the other distance sensor 4-2 passively receives. The distances L1 and L1+L2 can therefore be simultaneously determined in this combined operating mode.

The second distance sensor 4-2 then transmits and receives in its direct operating mode, while the first distance sensor 4-1 only passively receives, with the result that the distances L2 and L2+L1 can then be simultaneously measured.

The distance sensors 4-1 and 4-2 pass indirect measurement signals S3 and S4 to the control device 5. In this combined operating mode, both distances can already be determined from the two measurement signals from each measurement. In the first measurement in which the first distance sensor 4-1 actively transmits and receives and the second distance sensor 4-2 only passively receives, the distance L1 can thus be directly determined from the active measurement signal S1 from the first distance sensor 4-1 by halving, and this value can be subtracted from the total distance L1+L2 transmitted as the passive measurement signal S4 from the other distance sensor 4-2:

-   -   1. The distance sensor 4-1 transmits and both distance sensors         4-1 and 4-2 receive. The first distance sensor therefore         measures the distance D1=L1+L1 and the second distance sensor         4-2 measures the distance D2=L1+L2;     -   2. L1 is calculated from D1 by halving,     -   3. L2 is calculated from D2 and the L1 determined in step 2 by         subtraction.

The triangle 4-1, 10, 4-2 is therefore known, with the result that its height (height of the object 10 on the side d) can be determined as the object distance s between the object 10 and the sensors 4-1, 4-2 or the vehicle 1. According to FIG. 2, the lateral position p of the object 10 can then be determined, for example according to FIG. 2 as the section p between the height projection of the object 10 along the height s and the first distance the two distance sensors 4-1 and 4-2 and also with respect to the vehicle rear 2 (with the known position of the distance sensors 4-1, 4-2 on the vehicle rear 2) is thus known.

FIG. 3 shows another embodiment in which, in comparison with the embodiment in FIG. 1, the third, middle ultrasonic distance sensor 4-3 is additionally provided. In this embodiment, the emission angle regions (emission lobes) 8-1 and 8-2 of the two outer ultrasonic distance sensors 4-1 and 4-2 may possibly be rotated somewhat outwards since the middle region is detected by the distance sensor 4-3 which is additionally provided. A lateral rear region can therefore also be additionally detected here.

In the embodiment in FIG. 3, three direct distance measurements can be carried out. Indirect measurements are also possible, one of the distance sensors 4-1, 4-3, 4-2 alternately transmitting in each case and all three distance sensors 4-1, 4-2, 4-3 receiving, thus resulting in six indirect measurements and the distances L1+L2, L1+L3, L2+L3 each in turn being measured twice (in both directions).

A more comprehensive system of equations for determining the object distance s and the lateral width p can therefore be formulated in this embodiment.

In all embodiments, the object 10 may also be situated beside the vehicle 1. This lateral position of the object 10 can be detected according to the invention, the distance p being negative or greater than d in this case. Such objects can be directly rejected or indicated as being unobstructive by the algorithm.

In principle, the situation may occur in which an object 10 does not completely or symmetrically reflect ultrasonic waves 8-1 or 8-2 in all directions, for example on account of its material properties or, in particular, also the inclination of its surfaces. In FIG. 1 for example, the situation may thus occur in which the object 10 does not result in an echo in the two distance sensors 4-1 and 4-2. If, for example, the first distance sensor 4-1 transmits, the second distance sensor 4-2 may possibly not receive an echo or any reflected ultrasonic waves 11-1 or, conversely, only the second distance sensor 4-2 may receive reflected ultrasonic waves 11-1, but not the transmitting distance sensor 4-1 itself. In this last case in particular, it may not be possible at all to detect a signal in the conventional first operating mode. According to the invention, additional detection methods may be carried out in such cases and in other cases. These are, for example:

-   -   a radius estimation if only a direct echo has been received,         that is to say over the distance L1+L1 or the distance L2+L2.         The radius of the distance circle or of the distance sphere with         respect to the distance sensor which receives the direct echo is         therefore known;     -   triangulation of direct measurements if no indirect echoes have         been received. The distances L1+L1 and L2+L2 are therefore         measured, but no cross terms. In this case, conventional         triangulation from the individually measured distances L1 and L2         and the known sensor distance d is possible;     -   the determination of the object distance s on an ellipse if only         indirect measurements are possible. The indirect measurements         provide the sum L1+L2 of the two distances L1 and L2. All points         with this constant sum are on an ellipse, at the focal points of         which the distance sensors 4-1 and 4-2 lie. Such formation of an         ellipse may, in principle, be sufficient to detect a minimum         possible distance;     -   as back-up: both distance sensors 4-1 and 4-2 transmit and         receive at the same time in order to increase the emitted total         signal power. It is thus possible to achieve a total power which         is formed as the superimposition of the emission angle regions         (emission cones) 8-1 and 8-2 (the emission angle regions 8-1,         8-2, 8-3 in FIG. 3). This superimposition more strongly assumes         the shape of a parallel wavefront towards larger object         distances s. In this mode, the object distance s can only be         estimated; however, with larger object distances s in         particular, the measurement accuracy is no longer so relevant on         account of the unknown lateral position, in particular if s         becomes very large with respect to d. Estimations of larger         object distances s are therefore also possible.

The distance sensors 4-1 and 4-2 are advantageously synchronized via the control device 5 which outputs corresponding control signals or commands via the LIN bus 6. The control device 5 can thus output synchronization commands K1 to all distance sensors 4-1 and 4-2 in FIGS. 1 and 4-1, 4-2 and 4-3 in FIG. 3 via the bus 6, that is to say all sensors are addressed, the synchronization command K1 respectively containing a parameter for defining the transmitting distance sensor, and all distance sensors receiving, whereupon they output measurement signals S1 and S4 or S2 and S3 to the control device 5.

According to the invention, measurements in different planes and measurements across different planes are also possible. 

1. Environment monitoring system (3) for a vehicle (1), the environment monitoring system (3) having: at least two distance sensors (4-1, 4-2, 4-3) for detecting distances by measuring the propagation time of detection signals (9-1, 9-2), the distance sensors (4-1, 4-2, 4-3) each being in the form of a transmitting unit and receiving unit for the detection signal (9-1, 9-2, 11-1, 11-2) and, in a direct operating mode, each outputting detection signals (9-1, 9-2), receiving reflected components of the detection signals output by them and outputting active measurement signals (S1, S2) on the basis thereof, and a control device (5) which receives the measurement signals (S1, S2, S3, S4) from the distance sensors (4-1, 4-2, 4-3) and determines an object distance (s) of detected objects (10), characterized in that at least one distance sensor (4-1, 4-2, 4-3) can be additionally operated in an indirect operating mode in order to detect a detection signal (11-2, 11-1) output by another distance sensor (4-2, 4-3, 4-1) and reflected by the object (10) and in order to generate an indirect measurement signal (S3, S4).
 2. Environment monitoring system (3) according to claim 1, characterized in that it is in the form of a rear area monitoring system (3) of the vehicle (1) and is arranged on the rear (2) or in a rear region of the vehicle (1) in order to determine an object distance (s) between an object (10) in a rear area (7) behind the vehicle (1) and the rear area monitoring system (3) or the rear (2) of the vehicle (1).
 3. Environment monitoring system according to claim 1 or 2, characterized in that the detection signals are ultrasonic waves (9-1, 9-2) or radar waves or light beams which can be output by the distance sensors (4-1, 4-2) into an emission angle region (8-1, 8-2).
 4. Environment monitoring system (3) according to claim 3, characterized in that the emission angle regions (8-1, 8-2) of the distance sensors (4-1, 4-2) are directed rearwards and towards one another, with the result that they at least largely overlap in the rear area (7) behind the vehicle (1).
 5. Environment monitoring system (3) according to claim 3 or 4, characterized in that the emission angle regions (8-1, 8-2) of the distance sensors (4-1, 4-2) are substantially in a horizontal plane and detect an angle range of greater than 45 degrees, preferably at least 60 degrees, in the plane.
 6. Environment monitoring system according to one of the preceding claims, characterized in that each of the distance sensors (4-1, 4-2, 4-3) can be respectively operated in the following operating modes: the direct operating mode in which the distance sensor (4-1, 4-2, 4-3) emits a detection signal (9-1, 9-2) and detects the detection signal (11-1, 11-2) reflected by an object (10) in order to determine a distance (L1, L2) as the half-way mark of the distance covered in the detection time between the emission time and the receiving time, and the indirect operating mode in which the distance sensor (4-2, 4-1), without transmitting, passively receives the detection signal (11-1) emitted by another distance sensor (4-1) and reflected at an object (10) in order to determine a total distance (L1+L2) in an indirect measurement as the sum of the distances (L1, L2) between the two distance sensors (4-2, 4-1) and the object (10), the transmitting distance sensor and the receiving passive distance sensor (4-1, 4-2, 4-3) being synchronized in the indirect operating mode in order to align the emission time and receiving time, the further distance (L2) being able to be determined by the control device (5) by subtracting a distance (L1) determined in a direct measurement from a total distance (L1+L2) determined in an indirect measurement.
 7. Environment monitoring system according to claim 6, characterized in that a combined operating mode is provided, in which a first distance sensor (4-1) transmits and receives in the direct operating mode and at least one second distance sensor (4-2), without transmitting, simultaneously passively receives reflections (11-1) of the detection signal (9-1) emitted by the first distance sensor (4-1) in the indirect operating mode.
 8. Environment monitoring system according to claim 7, characterized in that the combined operating mode can be alternately carried out in such a manner that each distance sensor (4-1, 4-2) alternately transmits and receives and the other distance sensors (4-2, 4-1) simultaneously only passively receive.
 9. Environment monitoring system according to one of the preceding claims, characterized in that one or more of the following further operating modes can be carried out: radius estimation if only a single distance sensor (4-1, 4-2) receives a reflected detection signal (11-1, 11-2) in a direct measurement, triangulation by means of direct measurements, in particular if indirect measurements are missing, by determining the individual direct distances (L1, L2) between the distance sensors (4-1, 4-2) and an object (10) using a known sensor distance (d), back-up operating mode in which a plurality of, preferably all, distance sensors (4-1, 4-2, 4-3) simultaneously transmit and receive in order to increase the emitted total signal power and in order to estimate the object distance (s) between an object (10) and the rear area monitoring system (3), in the case of only indirect measurements, the determination of the object distance (s) on an ellipse which forms the set of all points with the same sum of distances to the distance sensors (4-1, 4-2).
 10. Environment monitoring system according to one of the preceding claims, characterized in that the control device (5) is intended to output synchronization signals (K1) to all distance sensors (4-1, 4-2, 4-3) in order to synchronize the distance sensors (4-1, 4-2, 4-3).
 11. Environment monitoring system according to claim 10 and according To claim 8 or 9, characterized in that the control device (5) is connected to the distance sensors (4-1, 4-2, 4-3) via a bus, for example a LIN bus (6), and outputs synchronization signals (K1) for the combined operating mode, the synchronization signals (K1) being addressed to all distance sensors (4-1, 4-2, 4-3) and containing, as a parameter, the statement of the distance sensor (4-1) transmitting in the direct operating mode.
 12. Environment monitoring system according to one of the preceding claims, characterized in that precisely two distance sensors (4-1, 4-2) are provided and are fitted to the left-hand and right-hand lateral rear regions of the vehicle.
 13. Environment monitoring system according to one of claims 1 to 11, characterized in that three distance sensors (4-1, 4-2, 4-3) are provided, a first distance sensor (4-1) of which is fitted to the left-hand lateral rear region, a second distance sensor (4-2) of which is fitted to the right-hand lateral rear region, and a third distance sensor (4-3) of which is fitted in the middle rear region of the vehicle.
 14. Environment monitoring system according to one of the preceding claims, characterized in that two distance sensors (4-1, 4-2) are respectively provided in a plurality of planes lying above one another.
 15. Method for determining an object distance (s) of an object (10), in particular using an environment monitoring system (3) according to one of the preceding claims, in which at least two distance sensors (4-1, 4-2, 4-3) each output detection signals (9-1, 9-2) and receive reflected components (11-1, 11-2) of the detection signals output by them in direct measurements, and receive reflected detection signals (11-2, 11-1) which were output by another distance sensor (4-2, 4-3, 4-1) and were reflected by the object (10) in indirect measurements, the object distance (s) being determined on the basis of at least one indirect measurement. 